Conventionally, tandem color image forming apparatuses in which different color toner images are formed by linearly arranged plural image forming sections, and the color toner images are transferred one by one onto an intermediate transfer medium, which is fed along the image forming sections, to form a combined color toner image on the intermediate transfer medium, have been used for producing multiple color images at a high speed. In each of the image forming sections of such tandem color image forming apparatuses, an electrostatic latent image formed on a photoreceptor serving as an image bearing member is developed by a developing device using a color toner to form a color toner image on the photoreceptor. The different color toner images thus formed on the photoreceptors of the image forming sections are transferred one by one onto the intermediate transfer medium so as to be overlaid to form a combined color toner image on the intermediate transfer medium, and the combined color toner image is transferred onto a recording medium such as paper sheets, resulting in formation of a multi-color image.
In order to remove residual toner remaining on the intermediate transfer medium even after the combined color toner image is transferred, cleaners to remove residual toner using an electrostatic force have been proposed.
For example, JP-4684617-B1 (i.e., JP-2006-119305-A) proposes a cleaner which electrostatically removes normally-charged residual toner, which is present on an intermediate transfer belt and which is charged normally (i.e., which has a charge with the same polarity as that of the toner used), and reversely-charged residual toner, which is also present on the intermediate transfer belt and which is charged reversely.
This cleaner includes a first cleaning brush roller to remove normally-charged residual toner from the intermediate transfer belt, and a second cleaning brush roller to remove reversely-charged residual toner from the intermediate transfer belt.
In addition, the cleaner includes a first counter roller, which is grounded and which is arranged so as to be contacted with the backside of the intermediate transfer belt while facing the first cleaning brush roller with the intermediate transfer belt therebetween, and a second counter roller, which is grounded and which is arranged so as to be contacted with the backside of the intermediate transfer belt while facing the second cleaning brush roller with the intermediate transfer belt therebetween.
A voltage having a polarity opposite to that of the normally-charged toner is applied to the first cleaning brush roller by a power source. In this case, a potential difference is formed between the first cleaning brush roller and the first counter roller, thereby forming an electric field such that the normally-charged residual toner on the intermediate transfer belt is electrostatically attracted by the first cleaning brush roller.
In addition, a voltage having the same polarity as that of the normally-charged toner is applied to the second cleaning brush roller by a power source. In this case, a potential difference is formed between the second cleaning brush roller and the second counter roller, thereby forming an electric field such that the reversely-charged residual toner on the intermediate transfer belt is electrostatically attracted by the second cleaning brush roller.
Thus, the normally-charged residual toner on the intermediate transfer belt is electrostatically attracted by the first cleaning brush roller, and therefore the residual toner is removed from the intermediate transfer belt. In addition, the reversely-charged residual toner on the intermediate transfer belt is electrostatically attracted by the second cleaning brush roller, and therefore the residual toner is also removed from the intermediate transfer belt.
In addition, the voltage applied to the cleaning brush roller is adjusted depending on the conditions of use. Specifically, the resistances of the intermediate transfer belt and the cleaning brush roller are typically predetermined. However, the resistances tend to vary due to variation of initial resistances of such members or when the members are used for a long period of time. When the resistance of the intermediate transfer belt or the cleaning brush roller falls out of the predetermined range and cleaning is performed under the normal conditions (i.e., at the predetermined voltage), defective cleaning is often caused.
The electrostatic cleaning performance of a cleaning brush roller highly correlates with the amount of the current flowing through the contact portion of the cleaning brush roller and the intermediate transfer belt. If the amount of the current can be maintained so as to fall in the targeted range, it is possible to maintain a high level of cleaning performance even when the resistances of the intermediate transfer belt and the cleaning brush roller vary.
In the image forming apparatus disclosed by JP-4684617-B1 (i.e., JP-2006-119305-A), the amount of the current flowing through the contact portion of the cleaning brush roller and the intermediate transfer belt is detected, and the setup voltage value stored in a memory is properly changed so that the targeted current flows through the contact portion of the cleaning roller and the intermediate transfer belt. It is described therein that occurrence of defective cleaning can be prevented because a voltage suitable for cleaning is applied to the cleaning brush roller even when the resistances of the intermediate transfer belt and the cleaning brush roller vary.